1. Field of the Invention
The present invention relates generally to image forming devices and, more particularly, to a composite torque transfer body and spline assembly to reduce jitter or banding in an image forming device.
2. Description of the Related Art
In one type of image forming devices, namely electrophotographic or laser printers manufactured and marketed by Lexmark International Inc., the developer unit of the cartridge is driven through an Oldham coupler, which allows for misalignment between the printer drive gear and the input drive gear of the developer unit. Oldham couplers have been employed for many years in drive connections of diverse machines for transferring torque or rotary power between two parallel but non-collinear and/or non-radially aligned rotating shafts. One drive connection employing an Oldham coupler in such manner in a laser printer is illustrated and described in FIGS. 3, 4A and 4B and column four of U.S. Pat. No. 7,130,562, assigned to the assignee of the present invention.
In the past, the drive gearbox of the printer has employed a gear driven retraction spline mechanism with all-plastic parts. These parts though present in FIGS. 4A and 4B of the above-cited patent are not numerically-identified therein. The retraction spline mechanism in the patent includes a gear, an input coupler at the drive input end of the Oldham coupler and a retraction spring disposed between the gear and the input coupler and surrounding spline parts on the gear and input coupler where mating and expansion and retraction of the gear driven retraction spline mechanism occurs. This design was found to be torsionally weak from the standpoint of stiffness of the drive system. The drive splines and bosses are known to deflect or oscillate under load. The torsional deflection has been shown to contribute toward creating a natural frequency in the drive system for the developer roller that, when excited, causes the developer roller to oscillation such that it produces banding at 0.7 mm spacing on the printed page. Mono or color printers seem to be sensitive to low frequency oscillations of the drive system. This can cause “fine line” jitter in the range of 0.5 to 2 mm, in the form of light and dark areas, on the printed page as the drive system oscillates.
In the past, composite gear designs have been proposed in which a polymeric torque transfer body is mechanically associated with a metal drive member in some manner with the objective of increasing the stiffness and torque transfer capabilities of the gear assembly. For example, a composite gear design of this general description is disclosed in U.S. Pat. No. 5,947,852. In this composite gear design, the metal and plastic hub and sprocket parts are clamped against a shoulder on a shaft that extends through the metal and plastic parts by bolts that pass through aligned holes in the metal and plastic parts and are then threaded into the shoulder on the shaft. While the approach of this patent might be sufficient in the types of applications contemplated therein, it would not appear to provide sufficient stiffening in an application such as the gear driven retraction spline mechanism of a printer.
Thus, there is still a need for an innovation that will stiffen the gear driven retraction spline mechanism enough to eliminate fine line jitter.